Carb's

[skids]

I First of all, I ain't no carb x'pert so take it for what its worth. I see you have a 78 model. Do you have the breather hoses connected to the airbox? Original configuration was to have two sets of T's for car pairs. One T was for air and one was for fuel. The upper hole is where the air T goes and the lower T is where the fuel T goes. OK, you probably don't have a problem with switching them by mistake. If you don't draw a vacuum through the breather hoses at high rpms, it can cause very rich conditions. Indy Rick found flooding conditions when a breather hose was plugged. Blane Hoops found that if the breather hoses are simply not connevted, overly rich conditions can occur. Hop that helps you out some.

Originally posted by strom
OK... some new info...

So, anybody who knows XS carb shrine, please HELP!

I noticed that under a bit higher revs, gas pours out of the hole on the right lower side on the back of each carb. What that hole does, is beyond me, but it floods the engine and whatever gas cant get there, runs to the airbox. I tried some K&N filters today after a carb session and I noticed my legs were sprayed with gas. Next I pulled the carbs again and blew some air with a compressor in the back side of the carb, and the piston thingie lifted and air-fuel mixture was sprayed out in front (I raised the front hatch-something-round thingies) and also gas came out of that hole in the rear (the right one). All four carbs do that.

Any ideas... don't be shy and help please!

LP

[strom]

I checked that and the air hoses seem nice and clean. Then I thaught, that the airbox could be plugged up where these hoses attach, so I ran the bike with the hoses disconnected, but not plugged. Same problem.

I checked what that gas-spitting jet connects to... its an air jet, which is pressed in and it connects above the main jet, just under the top. It should suck some air from behind the carbs and mix it with fuel comming from the main jet just before it's sprayed out on top. Next experiment will be to plug the holes, but the mixture will still be too rich IMHO, coz more fuel will be sucked up instead of some fuel and some air. It might work if I changed the main jets to the smallest possible size, but that's working around the problem and not quite solving it. I have no idea why fuel sprays out the back instead of air being sucked in. (did u notice I use the word "suck" a lot... it's a nice and pleasant word, but here... it sucks).

LP

[strom]

OK, one more possibility... the intake valves are closing too late...
will check... what do you guys think?

LP

[Ken Talbot]

No, Strom, I don't think intake valve timing has any thing to do with your fuel problem.

To reinforce Skid's advice, let's start with being absolutely sure the fuel line and the float bowl vent lines are connected properly. In this photo, the vent line to the airbox is connected to fitting A, and the fuel line from the tank is connected to fitting B.


Viewed from below with the float bowl removed, you can see where the bowl is vented to the atmosphere via A. Fuel comes in via B and its level in the bowl is controlled by the float and the needle valve.


Next is a view showing the jet that is leaking fuel.


Your observation is correct that this jet is connected to the main bore where the emulsion tube sits. You can just see the hole where it connects in this view.


This view shows how that air jet connects near the top of the bore that the emulsion tube sits in.


Now, my theory is that the only way fuel should come out of that air jet is if there is fuel under pressure above that hole next to the emulsion tube.

- If the fuel is coming in the correct fitting, and if the float and needle valve are working, the fuel level will never be that high, there will be no pressure head, and fuel will never come out the air jet.

- If the fuel is coming in through the wrong fitting, OR if the float and needle valve were stuck open, the fuel level could be much higher than the hole next to the emulsion tube, it would be under some pressure, and it could very well spit out of the air jet.

To check the float operation, try setting up your carbs on a small stand off of the bike. Attach a fuel supply to the correct fitting and let it sit for a few minutes to fill the float bowl to the correct level. Disconnect the fuel supply, then carefully remove a float bowl while keeping it upright. Ideally, when you ger it off, it will NOT be completely full of fuel. You should be able to float a float in the bowl and still and a bit of room. If gas pours out as soon as you loosen the float bowl screws, the level was much too high.

Keep us posted.

[strom]

OK, how much vacuum should there be at the intake rubbers when syncin carbs?

LP

[Ken Talbot]

At Sids's place last summer, we were getting approx 24cm of mercury on one of the bikes we were syncing:

[strom]

I wish I knew what was that in .bar
I checked one, just to see sth, coz I got only 2 meters and it was bout 0.1 - 0.12 bar.. when adjusted, but I don't know... would have to try on another bike.. with same adjustments to the meter, coz the needle went from 0 to 3 bars very fast before adjusting.. so I guess it should stay at bout 3... but Im not sure. And since i suspect the intake valves are closing too late, I tried to check what's going on... and I guess average is bout 1.2 or thereabout.
The rubber cam gaskets are leaking anyway, so I'll just whip off the valve cover and check the timing...

LP

[Ken Talbot]

Vacuum conversion calculator

Here's a link to a vacuum conversion calculator .

[strom]

Thanx.. helped a lot.

LP